Microtoroids are a form of integrated resonator that can have extremely high quality factors on the order of 100e6. The geometry and fabrication of microtoroids is taught in “Ultra-high-Q toroid microcavity on a chip” by D. K. Armani, T. J. Kippenberg, S. M. Spillane, K. K. Vahala, Nature 421, 925-928 (2003). In particular, microtoroids can be formed by thermally reflowing a material, so that the surface of the microtoroid can be shaped by surface tension and can be extremely smooth. This is the mechanism that allows such high quality factors, since scattering losses due to surface roughness are almost completely removed. High quality factors are very important for a number of applications such as optical time delay lines, narrow optical notch filters, or light generation in resonators via non-linear processes.
Microtoroids are a form of whispering gallery resonator in that they are essentially a waveguide forming a closed loop in which light can circulate. Due to the manner in which microtoroids are fabricated, they are also substantially planar, in that the height of the looped waveguide is constant relative to the chip surface and determined by the initial position of a thin film out of which the microtoroid is fabricated via a reflow process. In order to distinguish the waveguide forming the microtoroid from the waveguide to which the microtoroid is coupled to in this invention, the former is explicitly referred to as the looped waveguide in the following, while the latter is referred to as the integrated waveguide or coupled to waveguide.
An important drawback of these microtoroids is that they are extremely difficult to couple to integrated waveguides that are fabricated in the same chip than the microtoroids, i.e. to monolithically integrate them with waveguides. This is due to the fact that there is typically no mechanical connectivity between the portion of the thin film out of which the microtoroids are made by locally thermally reflowing said thin film and the remaining portions of said thin film elsewhere on the chip. For this reason, there is no mechanically supportive layer on which or in which a coupled to waveguide can be fabricated without this waveguide being trapped within the circumference of the microtoroid. The thin film out of which microtoroids are made by locally reflowing said thin film is referred to as the reflow film in the following. Monolithic fabrication of both the microtoroid and of the coupled to integrated waveguide would allow ultra precise positioning of the coupled to waveguide relative to the microtoroid, with the accuracy of lithographic feature definition, and would allow removing the high assembly costs incurred when assembling a microtoroid with a discrete fiber or waveguide, i.e. with a coupled to device that is not fabricated on the same chip.
A typical microtoroid is fabricated by starting with a thin-film out of a material that can be thermally reflown, such as a silicon dioxide film, on top of another material, such as for example a silicon substrate. A disk is first etched into the silicon dioxide thin film. The disk is than undercut by partially removing the silicon substrate from below the disk. This partial removal of the silicon is typically achieved with an isotropic etch such as is achieved by etching with gas phase XeF2. This results in a suspended disk located on top of a pedestal and attached to the rest of the chip via said pedestal. The rim of the disk is then locally reflown so that it forms a microtoroid. The reflow process is typically induced by heating with a C02 laser with a laser wavelength of 10.6 μm. The absorption coefficient of silicon dioxide is much higher than the absorption coefficient of silicon at 10.6 μm, so that the silicon dioxide film can be selectively heated relative to the silicon. Furthermore, the laser spot can be focused onto the disk, so that the silicon dioxide film can be locally heated in the vicinity of the disk. Finally, the silicon pedestal acts as a heat sink, so that the silicon dioxide heats up most at the rim of the disk, where the distance to the pedestal is the largest and the heat sinking the least efficient. This way, with the correct combination of C02 laser power, exposure time and focusing, the silicon dioxide film can be molten around the rim of the disk while remaining unmolten at the center. The silicon dioxide reflows at the rim forming an extremely smooth microtoroid with a circumference defined by the initial rim. Due to the reflow process, the looped waveguide is thicker than the reflow film out of which it is fabricated, thus forming a structure in which light can be guided.
Light is typically coupled to such a microtoroid from a tapered fiber, and the coupling coefficient between the fiber and the microtoroid tuned by adjusting the distance between the tapered fiber and the microtoroid. The tapered fiber is a suspended structure in the vicinity of the microtoroid in that it is surrounded by air.
Some attempts have been made to couple an integrated microtoroid with an on-chip waveguide. Such a structure is taught in “Silicon Microtoroidal Resonators with Integrated MEMS Tunable Coupler” by Jin Yao, David Leuenberger Ming-Chang M. Lee, and Ming C. Wu, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 13 (2007). In this structure a freestanding Silicon Waveguide is coupled to a non-inverted Silicon microtoroid resonator. This structure is fragile due to the free-standing waveguide, is complicated to fabricated, and it remains unclear whether it can be applied to microtoroids made out of silicon dioxide for which much higher quality factors can be achieved than for silicon microtoroids. It is essentially identical to the one described in the previous paragraphs in that the microtoroid is made by reflowing a disk and in that the coupled to waveguiding element is suspended in air and held in close proximity to the microtoroid.
Other shapes of microtoroids can be fabricated with the above-described method by etching other shapes into the silicon dioxide film, such as racetracks, ovals etc. This results in microtoroid type resonators for which a cross-section, typically a substantially circular cross-section, forms a closed loop mechanically connected to a non-reflown portion of the silicon dioxide film 6 and attached to the rest of the chip via a pedestal connected to said non-reflown portion of the silicon dioxide film. This non-reflown portion connected to the pedestal is inside the circumference of the microtoroid.